It turns out that these two examples are actually referring to the same thing: Saying “minus 6 dB per octave” might mean that the output signal at 200 Hz is only 1 2 For example 100 Hz and 200 Hz are one octave apart. Saying “minus 20 dB per decade” might mean that the output signal at 1000 Hz is only 1 10Īs large in voltage amplitude as the signal at 100 Hz.Īn octave is a 2X increase in frequency, just as it is in music. In future sections when we talk about frequency response of amplifiers and filters, we will sometimes consider numbers like “-20 dB/decade” or “-6 dB/octave”.Ī decade is a 10X increase in frequency, for example from 100 Hz to 1000 Hz. , then “-60 dBV” is simply a short way of saying “1 millivolt.”īe careful to track whether a decibel unit is relative or absolute, and whether the absolute reference represents a power or voltage level. In another example, “dBV” means “decibels relative to 1 Volt.” Therefore, -60 dBv represents “-60dB relative to 1 volt.” As -60 dB in voltage is a voltage factor of 10 − 3 However, it is often convenient to define the denominator as a specific unit-bearing quantity.įor example, “dBm” means “decibels relative to 1 milliwatt.” Therefore, +20 dBm represents “+20dB relative to 1 milliwatt.” As Watts are a unit of power, and +20 dB is a multiplication of 100X in power, then “+20 dBm” is simply a short way of saying “100 milliwatts.” We can add the decibel values to get +40dB overall voltage gain – a factor of 10 2Īny unit specified only as “dB” refers only to a relative factor between signals. If we connected two amplifiers together in series, each of which had 10x voltage gain, then each would contribute +20dB gain.

That’s why engineers speak colloquially about +3 dB or +6 dB to refer to a power gain of 2 or a voltage gain of 2, respectively. These exact decibel values for multiplication by 2Īre commonly truncated.

Logarithmic scale decibels